AZ237(1)_Hydraulic_Memo.pdf

Subject: AZ FLAP 237(1), Mormon Lake Road

From: Megan Frye, PE Hydraulic Engineer

FHWA‐Central Federal Lands Lakewood, CO

To: Micah Leadford Project Manager Lakewood, CO

This memorandum presents the field site observations, design analyses, and hydraulic recommendations for the proposed project along Mormon Lake Road, located 26 miles south of Flagstaff, AZ. Mormon Lake Road is primarily used as recreational route. Figure 1 provides a project location map for reference.

Figure 1. Mormon Lake Road ‐ Project Location Map

The County has expressed a desire to widen the paved width to provide multimodal access along Mormon Lake Road. Previous projects on Lake Mary Road have provided additional width for multimodal access. Creating a safer bike route along Mormon Lake Road is consistent with the County and National Forest’s vision of multimodal access throughout the Lake Mary corridor. The Central

Memorandum

Date: August 21, 2018

Mormon Lake Road

Federal Lands Highway Division, in cooperation with Coconino County and the Coconino National Forest, plans to widen 9.8 miles of Mormon Lake Road to a consistent paved width of 34 feet.

This memo addresses the hydraulic recommendations associated with the proposed project. The following items will be addressed:

Hydrologic and hydraulic criteria and computational methods

Hydraulic analysis and recommendations for major drainage structures.

DESIGN CRITERIA

The current and projected average daily traffic counts (ADT) on Mormon Lake Road are 758 and 1,126, respectively. The design criteria for Mormon Lake Road is based on the criteria defined for a low standard road by the Project Development and Delivery Manual. As such, the following design criteria apply:

Culvert Design (48” or greater)

Capacity: 25‐year flood

Headwater/Depth Ratio: 1.5 (< 48” culverts), 1.2 (≥48” culverts)

HYDROLOGIC ANALYSIS

A detailed analysis of the hydrology throughout the project was completed. The Watershed Modeling System (WMS v. 10.1) was used to delineate drainage basins at major culvert crossings. Figure 2 show the six major drainage basins coinciding with the major drainage structures.

Figure 2. Mormon Lake Road Major Drainage Basins

157+00

270+00

325+00

350+00

421+00

463+50

Table 1 details the drainage areas and major crossing description for each basin.

Existing Crossing Station

Drainage Area

(sq. miles)

Description

157+00 12.4 Double barrel, 10’x12’ RCBC

270+00 1.0 Double barrel, 36’ CMP

325+00 0.8 48” CMP

350+00 1.8 8’x4’ RCBC

421+00 1.3 42”x36” Squashed pipe

463+50 1.4 8’x4’ RCBC

Table 1. Major Drainage Basins

The peak discharges for each drainage basin were calculated using regression equations developed by Arizona State Department of Transportation. The USGS regression equations were developed using streamflow data primarily from Arizona, Nevada, and Utah. The drainages surrounding Mormon Lake Road fall within either Region 1 or Region 11. Region 1 represents all drainage areas with elevations greater than 7,500 feet and Region 11 represents the northeastern portion of the state. Because a portion of each drainage basin falls above 7,500 feet, the transitional zone equation identified in the USGS report (SITE) was used. The transitional equation is a weighted average of the flows calculated for the lower elevation regions and the flows calculated for the higher elevation regions.

The lower elevation regression equations (Region 11) use the drainage area and the mean annual precipitation to calculate peak discharge. Based on data collected by NOAA in Flagstaff, AZ, the mean annual precipitation for Mormon Lake is estimated at 22 inches. The higher elevation regression equations (Region 1) use the drainage area and the mean annual evaporation to calculate peak discharge. From the ADOT Hydrology Manual, the mean annual evaporation for Mormon Lake is estimated at 45 inches.

The peak flows calculated for each basin area presented in Table 2.

Existing Crossing Station

2‐yr 5‐yr 10‐yr 25‐yr 50‐yr 100‐yr

157+00 110 365 505 820 1180 1455

270+00 20 80 120 200 275 315

325+00 20 70 105 170 235 265

350+00 30 110 165 270 380 440

421+00 25 90 140 230 320 365

463+50 25 85 130 215 300 340

Table 2. Major Drainage Basin Peak Flows

HYDRAULICS ANALYSIS

EXISTING CULVERTS

Culverts were analyzed using HY‐8 culvert analysis software published by the Federal Highway Administration. The existing conditions at the culvert locations were analyzed to determine if the existing structures have adequate capacity.

Table 3 details the existing capacity check at each major culvert crossing for the 25‐year discharge. The results from the HY‐8 analysis are located in Appendix A.

Existing Culvert

Capacity Satisfied

HW Elev.

(ft.)

HW/D

Ratio

157+00 Double barrel, 10’x12’ RCBC

Yes 7119.5 0.5

270+00 Double barrel, 36’ CMP

No 7149.7 1.7

325+00 48” CMP No 7158.3 2.0

350+00 8’x4’ RCBC Yes 7147.0 1.4

421+00 42”x36”

Squashed pipe No 7141.7 1.3

463+50 8’x4’ RCBC Yes 7189.2 1.1

Table 3. Existing Culvert Capacity Check

PROPOSED HYDRAULIC RECOMMENDATIONS

The proposed road widening will result in culvert extensions throughout the project. While replacement of major culverts without adequate capacity may not be in the budget for the project, the capacity of the existing culverts was compared to the proposed condition at each location to determine if extending the existing culverts had adverse impacts to the surrounding area. Table 4 presents the existing and proposed headwater elevations for the 25‐year discharge along with details on the proposed extension.

The results from the HY‐8 analysis are located in Appendix B.

Existing Culvert Proposed Extension

Proposed HW Elev.

(ft.)

Change in HW Elev. (ft.)

157+00 Double barrel, 10’x12’ RCBC

None 7119.5 None

270+00 Double barrel, 36’ CMP

None 7149.7 None

325+00 48” CMP +10’ at outlet 7158.3 None

350+00 8’x4’ RCBC +14.5’ at inlet 7147.0 None

421+00 42”x36”

Squashed pipe None 7141.7 None

463+50 8’x4’ RCBC +8’ at outlet 7189.2 None

Table 4. Proposed Condition Headwater Comparison

Several of the culvert crossings fall with a FEMA‐regulated, Zone A floodplain. As such, the existing and proposed headwater elevations for the 100‐year discharge were compared to ensure the culvert extensions do not result in more than one foot of rise in the 100‐year water surface elevation. Based on the analysis using HY‐8, there is no impact to the regulated floodplain with the extension of the culverts.

Table 5 details the existing and proposed headwater elevations at the culverts within the regulated floodplain.

Existing Crossing Station

Existing Culvert Proposed Extension

Existing HW Elev. (ft)

Proposed HW Elev. (ft)

157+00 Double barrel, 10’x12’ RCBC

None 7122.0 7122.0

325+00 48” CMP +10’ at outlet 7158.6’ 7158.6’

350+00 8’x4’ RCBC +14.5’ at inlet

7151.1’ 7151.1’

Table 5. Comparison of 100‐year Headwater Elevations

350+00 – Proposed Outlet Protection The existing 8’x4’ reinforced concrete box does meet capacity requirement. The slope of the box is flat and the outlet is perched approximately 4 feet above the channel. The outlet of the box also has several irrigation intakes on the apron. Because of the irrigation systems, the recommendation was to widen the roadway only on the upstream side and to leave the outlet of the box as is. Figures 4 and 5 show the existing conditions at the box culvert.

Figure 4. Looking at inlet Figure 5. Looking at outlet

The existing culvert will be extended 14.5 feet at the inlet. Set the inlet invert elevation to 7141.5 feet to maintain the flat slope. Install a riprap apron at the outlet using Class 4 riprap. See Figure 6 for the detail of the riprap apron. A size analysis for the riprap can be found in Appendix C.

Figure 6. Riprap Detail

REFERENCES

ADOT. “Arizona Department of Transportation Highway Drainage Design Manual” Volume 2 Hydrology, Second Edition, 2014.

FHWA (2007) “Project Development and Design Manual for Central Federal Lands Highway Division” Chp. 7. Federal Highway Administration (FHWA).

FHWA HY‐8 v. 7.5 (October 2017).

FHWA Hydraulic Toolbox v. 4.3 (October 2017).

APPENDIX A – HY‐8 EXISTING CONDITIONS

157+00 – CULVERT SUMMARY TABLE (25‐YR FLOW HIGHLIGHTED)

270+00 – CULVERT SUMMARY TABLE (25‐YR FLOW HIGHLIGHTED)

325+00 ‐ CULVERT SUMMARY TABLE (25‐YR FLOW HIGHLIGHTED)

350+00 ‐ CULVERT SUMMARY TABLE (25‐YR FLOW HIGHLIGHTED)

421+00 ‐ CULVERT SUMMARY TABLE (25‐YR FLOW HIGHLIGHTED)

463+50 ‐ CULVERT SUMMARY TABLE (25‐YR FLOW HIGHLIGHTED)

APPENDIX B – HY‐8 PROPOSED CONDITIONS

325+00 ‐ CULVERT SUMMARY TABLE

(25‐YR & 100‐YR FLOW HIGHLIGHTED)

350+00 ‐ CULVERT SUMMARY TABLE

(25‐YR & 100‐YR FLOW HIGHLIGHTED)

463+50 ‐ CULVERT SUMMARY TABLE (25‐YR FLOW HIGHLIGHTED)

APPENDIX C – HYDRAULIC TOOLBOX RIPRAP

350+00 – OUTLET PROTECTION